Wide latitude heat-sensitive copy-sheet and method of making

ABSTRACT

HEAT SENSITVE COPY SHEET CONTAINING BOTH SILVER SOAP AND FERRIC SOAP TOGETHER WITH A REACTANT FOR BOTH, SHOWS INCREASED EXPOSURE LATITUDE IN THERMOGRAPHIC COPYING PROCESS.

United States Patent 3 682,684 WIDE LATITUDE HEAT-SENSITIVE COPY-SHEET AND METHOD OF MAKING Donald J. Newman and Donald J. Williams, White Bear Lake, and John R. Berg, St. Paul, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn. No Drawing. Filed Mar. 10, 1971, Ser. No. 123,106 Int. Cl. B41m 5/18 U.S.'Cl. 11736.9 7 Claims ABSTRACT OF THE DISCLOSURE Heat sensitive copy sheet, containing both silver soap and ferric soap together with a reactant for both, shows increased exposure latitude in thermographic copying process.

This invention relates to heat-sensitive copy-sheets. In one important aspect the invention relates to copy-sheets in transparent film form with which projection transparencies of halftone originals may be prepared by thermographic copying processes.

The thermographic process as first described in Miller US. Pat. No. 2,740,896 involves brief exposure to intense radiation of an original in heat-conductive contact with a heat-sensitive copy-sheet. In such processes the heat pattern produced at wide lines or blocky image areas may build up more rapidly than at narrow lines of areas of fine detail, resulting in blurring at the former or in partial copy at the latter areas when the copy-sheet employed is deficient in exposure latitude. Copy-sheet constructions based on silver soap reactants, as described in Owen US. Pat. No. 2,910,377, produce dense black images but the exposure must be carefully regulated in order to obtain a useful degree of latitude. Increased latitude is obtained with copy-sheet constructions based on ferric soaps as described in Miller et al. U.S. Pat. No. 2,663,654, but even with these sheets it is found difficult to obtain fully satisfactory rendering of both fine and heavy lines or patterns.

Surprisingly, it has now been found that a combination of silver and ferric soap will produce a copy-sheet composition having far superior exposure latitude as com pared to either used separately. The discovery makes possible the accurate rendering of both fine detail and wide area images on a single sheet, and has resulted in the ability to produce excellent full detail projection transparencies of halftone and other difiicult-to-reproduce originals.

The silver soap heat-sensitive copy-sheets form a visible image through an oxidation-reduction reaction. A different reaction, probably a chelation reaction, occurs in the ferric soap coatings. Separate co-reactants may be employed, but it has been found possible to use the same co-reactant for both of the metal soap reactants. Compounds such as pyrogallic acid, catechol, protocatechuic acid, azelayl pyrogallic acid, azelayl bispyrogallic acid, methyl gallate, behenoyl pyrogallol are particularly effective. Each of these compounds will be seen to a polyhydric phenol. Mixtures of these and other co-reactants for the metal soaps may be used, e.g. to obtain images of preferred shades of color. Toners such as phthalazinone are known to improve the silver image in copy-sheets based on silver soaps and are found desirable in the prodnets of this invention, e.g. in amounts of about 15-20 percent of the weight of the silver soap. Polymeric filmforrning binders, non-fusing at the temperatures employed in copying, will ordinarily be included in amounts sufficient to form a smooth well-bonded coating. The compositions may be applied to paper or other nontransparent 3,682,684 Patented Aug. 8, 1972 ice backings and when thus employed may contain pigments, fillers and other additives. Particularly desirable results are obtained using clear transparent coatings on transparent film backings, the resulting copies then serving admirably as projection transparencies for use on overhead projectors.

The following specific examples, in which all proportions are given in parts by Weight unless otherwise indicated, will serve further to illustrate but not to limit the invention.

EXAMPLE 1 A first coating composition is prepared by mixing together 20 parts of a 20% dispersion of ferric stearate in ethanol, 20 parts of a 20% dispersion of silver behenate in ethanol, 0.66 part of phthalazinone, 66.66 parts of a 15% solution of polyvinyl acetate in acetone, and 88.5 parts of acetone. The mixture is coated on two mil polyethylene terephthalate polyester film at a coating Weight, after drying, of 0.55 gram per sq. ft.

A second coating composition, prepared by dissolving 20 parts of cellulose acetate butyrate and 4 parts of methyl gallate in 180 parts of methylethyl ketone, is applied oer the first coat at a coating weight, after drying, of 0.30 gm./sq. ft. The two coatings provide a thin visibly heatsensitive layer.

The coated film is clear and transparent, with a faint buff color. A portion is placed in face-to-face contact with an original having black printed characters on a white paper backing. Brief exposure through the film to intense radiant energy in a thermographic copying machine results in the reproduction of the image areas on the film. The image density is approximately 1.1. Images having a density above about 0.6 are effective in providing projection images which are easily visible when shown in a lighted room.

Another portion is used to make a copy of a printed halftone original by a similar procedure, The dark image areas reproduce as a dense black; the reproductions of lighter areas are of much lower density and have a slightly purple cast.

EXAMPLE 2 A silver soap dispersion is prepared by mixing together 60 parts of silver behenate, 240 parts of ethanol, parts of polyvinyl acetate, 11.25 parts of phthalazinone, and 1089 parts of acetone.

A ferric soap dispersion is similarly prepared by mixing together 60 parts of ferrice stearate, 240 parts of ethanol, 75 parts of polyvinyl acetate, and 1100 parts of acetone.

The two dispersions are mixed together in different proportions as hereinafter tabulated. The mixtures are coated on one mil polyester film through a coating orifice of four mils, and the coatings are dried in an oven at temperatures not higher than about 200 F. Each sample is then further coated with a composition containing 9.8 parts of cellulose acetate butyrate and 2.0 parts of methyl gallate in 88.2 parts of methylethyl 'ketone, applied through a three mil orifice, and the sheet is again dried. The several samples are imaged thermographically by being placed against an original, printed with both wide and narrow lines, which is then exposed through the sample to intense radiant energy for decreasing periods of time along the length of the lines. The test is conveniently carried out in a thermographic apparatus wherein the rate of movement of the copysheet and printed original past a line source of intense radiation is uniformly increased, thereby providing a graphical representation of exposure latitude. Under unduly prolonged exposure the copy is blurred, the heavy lines in particular being broadened and showing rough wavy edges. With too short an exposure the density of the copy is decreased and the finest lines of the original do not reproduce. The range of ex- TABLE I [Effect of FezAg ratio on relative exposure latitude] Percent Ag soap 20 30 40 50 60 70 30 90 100 Percent Fe soa 100 90 80 70 60 60 40 30 20 10 0 Latitude 33 37 40 46 47 52 50 51 50 18 Surprisingly, the latitude of the mixtures containing 10- 80% of silver soap is increased to a value far in excess of that obtained with either of the soaps alone.

EXAMPLE 3 One mil polyester film is provided with a dried first coating of a mixture containing equal proportions of ferric stearate and silver behenate together with toner and binder, as described in connection with Example 1. Portions of the film are then separately coated with compositions containing two parts of different co-reactants and 9.8 parts of cellulose acetate butyrate in 88.2 parts of methylethyl ketone, applied from a three mil coating orifice and again dried. The same coatings are applied over first coatings separately containing the two metal soaps, as controls. The several samples are tested for exposure latitude as previously described, with results as indicated in the following tabulation.

TABLE II [Eflect of co-reactant on relative exposure latitude] Relative exposure latitude (Jo-reactant Ferric Silver Mixture Azelayl pyrogallic acid 23 12 27 Protocntechulc acid- 33 14 38 Pyrogallic acid 24 11 46 Methyl gallate 17 13 43 Azelagl bis pyrogallic acid 23 16 29 Catec 01 32 1O 45 Behenoyl pyrogallol 38 60 EXAMPLE 4 the thermographic copying process. The image areas are of a more nearly uniform appearance than in the case of 4 the film of Example 1, the image color being gray rather than purple at the lighter areas.

What is claimed is as follows:

1. A heat-sensitive sheet material including a thin visibly heat-sensitive layer having wide exposure latitude and comprising: a mixture of ferric and silver soaps of long chain fatty acids wherein said silver soap represents between 10 and percent by Weight of said mixture; at toner for the silver image; and a phenolic co-reactant for said soaps.

2. Sheet material of claim 1 wherein said layer contains a polymeric film-forming binder and is supported on a transparent carrier film and wherein said sheet material is clear and transparent.

3. Sheet material of claim 2 wherein said ferric and silver soaps are present in approximately equal proportions.

4. Sheet material of claim 3 wherein said ferric soap is ferric stearate, said silver soap is silver behenate, and said toner is phthalazinone.

5. Sheet material of claim 4 wherein said phenolic coreactant comprises behenoyl pyrogallol.

6. Sheet material including a thin flexible backing and having a first coating comprising a. mixture of 10-80 parts by weight of silver soap of long chain fatty acid and correspondingly -20 parts by weight of ferric soap of long chain fatty acid, and overlying said first coating, a second coating comprising at least one polyhydric phenolic coreactant, heat reactable with said soaps to produce a colored product.

7. Method of making a heat-sensitive copy-sheet having wide exposure latitude comprising blending together, in a solution of a polymeric film-forming binder in a volatile liquid solvent, 10-80 parts by weight of silver soap of long chain fatty acid and correspondingly 90-20 parts by weight of ferric soap of long chain fatty acid, together with about 15-20 parts by weight of phthalazinone for each parts of said silver soap, and applying said blend to a backing, and applying an overlying coating containing at least one polyhydric phenolic co-reactant for said soaps thereto.

References Cited UNITED STATES PATENTS 2,637,657 5/1953 Ozols l17-36.8 2,910,377 10/ 1959 Owen 117-36.9 2,999,035 9/1961 Sahler 117-368 3,107,174 10/1963 Wartmau 117-36.8 3,168,864 2/ 1965 Brandi et al 11736.8 3,185,585 5/ 1965 Baumann et a1 117-36.8 3,619,237 11/1971 Leclair 117-368 MURRAY KATZ, Primary Examier US. Cl. X.R.

10626; l17138.8 F, UA, 157; 260-41 C 

